RFR 8021883 j.u.Random/RandomStream.java test needs more robust timeout duration

[Paul Sandoz]

Hi,

http://cr.openjdk.java.net/~psandoz/tl/JDK-8021883-random-stream-test/webrev/

This patch updates tests for ThreadLocalRandom streams and increases the timeout duration. Although the original should be enough for most cases in certain virtualized and emulated environments it might not be. I also took the liberty of consolidating the code and sprinkling stream and CompletableFuture dust on things.

Paul.